Microphonic caps?

[nee]

Thought I should start a new thread now I have the hum problem sorted!

After a little guitar playing with the new TW, initially I thought V1 was microphonic, but I swapped it out for a known good one and the problem was still there.

In the course of some testing, I pulled V1 and chopsticked around. The .002 uf coupling cap between V1 and V2 causes a buzz as the wooden chopstick approached it, and makes metallic banging noises through the speaker if I tap. Is this likely to be the cap or something else in the chain? It's an orange drop (actually 2 x .001s in parallel).

[nee]

Doing some more fiddling around...

I removed the orange drops and tacked in a different cap (an old SRC): same problem. The cap is microphonic, and V1 is heavily microphonic.

I know the valve is good. Anyone have any ideas?

[jjman]

It’s possible for a “non-microphonic” tube to be microphonic in V1 of a TW amp since there is so much gain. I have tubes that reveal this slight microphony only in that position of that one amp.

Does the microphony increase with the volume knob?

[nee]

Does the microphony increase with the volume knob?

No. With volume down, tapping the coupling cap (usually designated C8 in the drawings) or V1 results in V1 microphony.

I've been reading a few threads in the meantime. Some suggest shielded wire to V1 pins 2 and 7, which I already have. I also have a 33K resistor right on pin 7 from the jack input. The shielded wire I'm using is genuine Fender, so either that's OK or that's the problem!

[fishy]

I think you are discovering the reasons why these amps can be a bit touchy. Lead dress can have a big influence on the microphonic tube. Rooster has put forward a solution which seems to work very well. The thread eludes me right now. Selecting a primo tube for V1 still helps.
I solved much of my microphonic V1 issues with lead dress and really thick PVC insulated wired.

The cap discovery seems about right to me also. Applying some RTV to the cap will deaden it some but I do not think your discovery is out of the norm or at least not to me.

[dimitris]

How is wired your bright cap? If you have the cap soldered to the middle lug of the volume pot and you are switching the outer side, try to do the opposite. This thing solved some similar microphonic problems I had in a D clone.

[El_Martin]

Hi!

When I built my first TW (TeleWreck), I observed the same with a ceramic cap. The 001 || 001 coupling cap combination. Tried several caps. Seems to my like the quality of the soldering does have a similar effect.
Currently there are the 001 || 001 caps again in the TeleWreck without extreme microphonics.
Maybe some caps (wires w. solderings) act not much unlike condenser micros

Ciao
Martin

[Structo]

Another trick I read about is to take some heat shrink tubing a little wider than the preamp tube and cut a length that is about 2/3 the length of the tube.
Slip it over and shrink it so the top 2/3 of the tube is covered leaving the top exposed.
This way you can still use the shield can but the tube is dampened reducing microphonics.

This is the thread where Rooster describes the tube isolator that he said works well.

Of course the other solution is to put a lower gain tube in V1.

https://tubeamparchive.com/viewtopic.php?t=8561

[nee]

I think you are discovering the reasons why these amps can be a bit touchy... I do not think your discovery is out of the norm.

I think that's it when all's said and done. Given that this is my first amp build, and reading horror stories here about squealing, oscillation, RF noise, buzzes, crackles and farts, I couldn't be happier. This is one big beast of an amp, quite unlike anything I've ever played through before.

[nee]

How is wired your bright cap?

I don't have the bright cap installed.

[hitchcaster]

you could try what i call audio "duct tape".... which is blue tack on the cap to see if that gets rid of the microphonics... or some kind of potting.. sounds like its a different problem though, and you'll be just treating the symptom US healthcare style
good luck

[Structo]

On my D amp there is a large orange drop which is part of the local feedback on V1. It was kind of hanging out there so I put some clear silicone caulk under it to secure it and to also dampen it from vibrations.
Seems to work well.

[nee]

I've replaced the leads to V1 pins 2 and 7. They themselves were a little microphonic (they were genuine Fender parts!)

V1 is still very microphonic though, so I think I'll try blue tack or silicon around the offending "C8", and some sort of damping ring under the tube socket. I think we have a silicon baking pan that may do the job!

It still seems odd to me that C8 is very microphonic when tapped, but no other component is - apart from the V1 socket itself.

[Structo]

Don't you think that is because of the mechanical connection to the tube?

I mean, I do know a cap itself can be microphonic (condenser mic?) but I know when I have chop sticked around in different amps it's not that uncommon to fine just about everything surrounding a V1 with a 12ax7 in it will make noise.

I just don't like it when it is the type of micropnonics that ring and turn into feedback.

[nee]

Don't you think that is because of the mechanical connection to the tube?

Well, the whole chassis is pretty lively no matter where you tap it. However, the noise from tapping the cap is almost as bad as tapping the tube itself - very loud. Tapping the plate resistor, which is connected to the same place as the cap, isn't nearly as loud.

So maybe it is the "condenser mic" theory. It just struck me as odd that whatever make or kind of cap I put there, it made the same noise, which implies it is mechanical connection, but the resistor doesn't make a noise, which implies that it isn't.

Which is just a long-winded way of saying: any cap connected to the tube is noisy; anything else connected to the tube isn't.